There are several circumstances where it is desired to measure the change in concentration of chemical substances in a solution in a dynamic fashion. In the wine making industry the levels of sugar and alcohol are measured in order to determine the extent to which the fermentation process should be continued. Similarly with the beer industry. Traditionally such industries measure the level of sugar by measuring the density of the solution by refractometry. Such a practice requires opening of the fermentation vat, with attendant risk of contamination, removing a sample and measuring. The method entails determining the refractive index of the solution which gives an indication of the density. A concordance table is consulted to determine the quantity of sugar. A calculation is then made to determine the reduction in sugar concentration relative to the starting concentration and from that a measure of the alcohol concentration is calculated. This calculated value gives only an approximation, requires the recordal of data manually and a direct sampling from the vat.
There have been suggested a number of approaches to measuring concentrations of various molecules, such as sugars in a dynamic fashion, and these include the use of an osmotic cell such as disclosed in U.S. Pat. No. 5,005,403 in the name of Steudle et al. This arrangement is quite complex and the document makes no disclosure of measuring the cumulative effects of multiple chemicals.
Another approach to measuring the presence of glucose is to use a specifically reactive, pH sensitive, hydrogel which has a glucose reactive linked enzyme the product of which reaction leads to changes in pressure emanating from the hydrogel. (see U.S. Pat. No. 6,268,161 to Han et al.,) This is a relatively complex arrangement suitable for specialised uses.
Hydrogels have been used in the past for monitoring. One such disclosure is WO 00/37935 by the present inventor whereby hydrogels held under a compressive force have been used to measure the matrix potential of soil.